This invention relates to the preparation of polyurethane foams. This invention further relates to the use of certain organotin catalysts to prepare urethane foams that exhibit improved hydrolytic and oxidative stability relative to products obtained using other organotin catalysts.
Flexible polyurethane foams are widely used in the manufacture of furniture, particularly seat cushions, and as packaging material for delicate instruments and other articles that are susceptible to damage during handling and transit. If the foam is incorporated into a piece of furniture or other durable product, the foam must withstand exposure of several years or longer to elevated temperatures, atmospheric oxygen, or both without undergoing significant degradation as evidenced by a gradual loss of structural strength, which may culminate in disintegration of the foam.
It is well known to prepare cellular polyurethanes by reacting polyols containing two or more reactive hydrogen atoms, as determined by the Zerewitinoff method, with polyfunctional isocyanates in the presence of a polymerization or gel catalyst and a blowing agent such as water or a relatively low boiling fluorinated hydrocarbon. A surfactant is often included in the reaction mixture to obtain the desired small, uniform cell size within the foam.
U.S. Pat. No. 3,620,985 discloses that both divalent and tetravalent tin compounds are effective gel catalysts for cellular polyurethanes. The divalent tin compounds, exemplified by stannous salts of carboxylic acids, such as stannous octoate, are so susceptible to oxidation that they decompose readily in the presence of air. Special handling and storage of these stannous compounds is therefore required to retain their activity as catalysts. Compounds of the formula R.sub.a SnX.sub.4-a wherein R is typically butyl or other alkyl radical containing between 2 and 20 carbon atoms and X is halogen or other anionic radical are less than satisfactory for preparing flexible foams that are exposed to atmospheric oxygen, elevated temperatures, i.e. above about 50.degree. C. or both over extended periods of time. Under these conditions the foams may lose resilience and structural integrity, sometimes to the extent that they disintegrate when compressed. This is particularly true for those foams derived from polyols that are reaction products of propylene oxide and glycerine. These polyols may also contain end groups derived from ethlene oxide. Polyurethanes derived from polyols that contain side chains resulting from the graft polymerization of acrylonitrile, styrene or other vinyl monomer onto a poly (propylene oxide) backbone are usually less susceptible to oxidative and thermal degradation, and are therefore employed to prepare high resiliency foams. However, in the presence of conventional organotin gel catalysts even these products undergo a significant decrease in residiency following prolonged espoxures to oxygen and/or heat. Measuring the loss in resiliency that occurs during heating constitutes a commercially accepted test procedure for evaluating cellular polymers. The experimental procedure for conducting this test is available as ASTM Test D-1564, suffix A, also referred to as the indent load deflection test, and is described in the accompanying examples.
An objective of this invention is to improve the resistance to oxidative degradation of polyurethane foams prepared using tetravalent organotin compounds as the gel catalyst.
Surprisingly it has now been found that certain organotin gel catalysts wherein the hydrocarbon radicals bonded to the tin atom are methyl, allyl, phenyl or benzyl are unique in that they do not adversely affect the oxidative stability of flexible polyurethane foams.